Its a L1A1 gas block and tube and pistion. I have enough blocks for about 9 differant barrels. Wont be using the tube or the Pistion although I have considered it. Not sure the smaller Id will work correct. I am considering experimenting with pision sizes VS gas port sizes for the optium combination. Dazinga arms or some thing like that has larger OD standard looking GB,s for around $60 or so. Or make your own like i did on the Varmit build.